ABSTRACT. Nude mice were infested with bovine Neospora by intraperitoneal inoculation of the brain and spinal cord from an aborted bovine fetus due to neosporosis. Inoculated mice showed severe emaciation and tetraplegia at about 2 to 4 months post-inoculation. Histopathologically, polyradiculoneuritis and peripheral neuritis were the major findings, and lesions in the central nervous system were located at periventricular and submeningeal areas of the cerebrum or at the white matter around the roots of radices of the spinal cord. These findings may suggest the protozoa inocualted into the abdominal cavity invaded the spinal cord via the spinal nerve and later reached the brain through the cerebrospinal fluid. -KEY WORDS: Neospora, nude mouse, polyradiculoneuritis.J. Vet. Med. Sci. 59(10): 947-948, 1997 immunohistochemical examination, anti-N. caninum goat serum (VMRD Inc., Wash., U.S.A.) was used at a dilution of × 4,000 as the primary antibody. Antigen was detected by using a biotinylated rabbit anti-goat secondary antibody (Vector Laboratories) and the ABC Elite kit (Vector Laboratories) with 3,3'-diaminobenzidine as chromogen. Body weight of 4 mice inoculated with bovine N. were 18.1 ± 3.07 g (28.5 ± 0.85 g for control mice) and they showed tetraplegia at about 2 to 4 months post-inoculation (Fig. 1). The mice were killed at terminal stages, and 3 of 4 mice showed severe swelling of the systemic lymph nodes on necropsy. Microscopically, all mice showed polyradiculoneuritis (Fig. 2). The lesions were characterized by the swelling of Schwann cells, mild or moderate infiltration of neutrophils, lymphocytes and macrophages, and degeneration of axons and myelin. The changes were the most intense in the radices and major trunks of spinal nerves of lumbosacral segments, and in the nerve fibers of skeletal muscles, and were mild in the cranial nerves. A small number of tachyzoites were seen in the endoneurium of spinal radices. Cerebral lesions were found in one mouse, in which a number of necrotic foci were located at periventricular and submeningeal areas. Spinal ganglia were Infected mouse is emaciated and shows tetraplegia. Neospora (N.) caninum is an apicomplexan parasite that can cause paralysis and death in congenitally infected dogs [3]. A N.-like organism has been reported as a cause of abortion and neonatal mortality in cattle, and the disease of cattle is the significant cause of economic and reproductive loss to the livestock industry [5]. Bovine N. has been isolated in cell line [1,8]. The mice inoculated with N. caninum developed acute pneumonia, polymyositis, encephalitis and ganglioradiculoneuritis [6,7]. In this report, we describe the distinctive pathological changes of nude mice inoculated with bovine N. and discuss the entry route of the parasite to the central nervous system (CNS).Inoculum: The brain and spinal cord of aborted bovine fetus of approximately 8 months gestational age were obtained at Ibaraki prefecture, Japan. On histopathological examination of the fetus, mild nonsuppurative me...